1. Field of the Invention
The present invention relates to an image processing apparatus suitable for use in electronic file, fascimile, reader or digital copier.
2. Description of the Prior Art
It has been proposed to photo-electrically read a document image by means of an image sensor such as a CCD or A-Si device, binarize the read signal by a threshold level, and reproduce an image by a binary printer (dot printer) such as a laser beam printer or ink jet printer. It has also been proposed to compress the binary read signal by a modified Huffman encoder for transmission or storage in a magnetic or optical memory.
In one method of converting the document image to the binary signal, the read signal is compared with a fixed threshold level to discriminate density (binarize), and in another method, each pixels in a predetermined pixel block (e.g. 4.times.4 pixels) is compared with a plurality of different threshold levels corresponding to a density between white and black and some pixels in the block are determined as black (or white) so that multi-tone image reproduction is attained.
The dither method is an effective technique because it allows multi-tone image reproduction with a relatively simple construction. However, the dither method has a drawback in that the resolution of the reproduced image is low. The fixed threshold binarization method provides a high resolution but cannot attain the tonality reproduction of the dither method.
In order to prevent the reduction of the resolution and attain the tonality reproduction, it has been proposed to divide the document image into a multi-tone reproduction area and a high resolution reproduction area and automatically or manually select the binarization processing in accordance with the content of the image such that the former area is processed by the dither method and the latter area is processed by the fixed threshold binarization method.
However, if image data binarized for reproduction on a sheet is to be stored in a file as it is, a problem may arise. The binary data for the reproduction on the sheet to represent exact densities are represented by a plurality of dots per pixel and cause the reduction of the compression factor in the dither method. Accordingly, a larger area of the file is required and the number of images stored in one image file such as a magneto-optical disk is reduced.